The High Energy Light Isotope eXperiment (HELIX) is a NASA-sponsored project to fly a complex particle physics instrument on a long-duration balloon in Antarctica, at altitudes of about 120,000 ft where cosmic rays can be observed before they are destroyed in interactions with the atmosphere. The aim of the project is the difficult measurement of various isotopes in the flux of cosmic ray particles arriving at earth. In particular the relative proportion of 10Be and 9Be isotopes at energies between 1 and 10 GeV/nucleon will provide crucial clues on the mechanisms of cosmic ray production and transport in the Milky Way Galaxy. The instrument is challenging as it uses a powerful superconducting magnet, a tracking chamber, timing scintillators and a ring-imaging aerogel detector, with stringent requirements on weight, power, ruggedness for flight under very difficult conditions. The payload is expected to fly in 2018.